Previously we developed a specific radioimmunoassay for the BB isoenzyme of creatine kinase (CK-BB). Currently, we are applying this assay to the measurement of CK-BB in human serum and other body fluids, human tumor homogenates, and cell cultures. Serum levels of CK-BB are an early and sensitive marker of acute myocardial infarction. Since serum levels of CK-BB are also elevated in the presence of a number of cancers, we have investigated the role of CK-BB as a tumor marker in patients with cancer of the lung, gastrointestinal and genito-urinary tracts. It appears that CK-BB may be most useful in patients with small cell carcinoma of the lung.